The selection of characteristic impedance effects the capacitance per unit
length of the trace. Forgetting about board geometries constarints, driver
impedance
issues, and crosstalk coefficients, the problem with too high a value is
that the capacitance per unit length gets very low. This means that
non-uniformly distributed vias and loads create impedance discontinuities
which in turn produce reflections. Reflections create EMI radiation.

Lets say I have a trace on a board between two devices. There is nothing
that determines what the characteristic impedance of the trace should be.
That is, there are no device considerations, no space considerations, no
power considerations, etc. that would favor one value of characteristic
impedance over another. It is my opinion that as long as the characteristic
impedance is constant over the length and properly terminated, the choice
of impedance value in this situation is completely arbitrary.

Does anyone (having accepted the assumptions above) have a different
opinion? In particular, are there any EMI reasons that would favor one
value of characteristic impedance over another? (But don't tell me the
assumptions above are wrong! That's not the question!)

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